Assessing Estimates of Low-Level Supercell Circulation and Areal Expansion Rate Diagnosed from Doppler Radar Data: Simulation Study

Wood and Davies-Jones (2015) simulated Doppler velocity fields that a Doppler radar would see as it scanned an axisymmetric vortex embedded in a convergent axisymmetric flow. We gauged the detection and measurement of significant circulation around and areal expansion rate of a curve as a method for giving advanced warnings of tornadoes. Circulation (rate of areal expansion) is the line integral around a closed curve of velocity tangential (outward normal) to the curve. Circulation and areal expansion rate (the negative of areal contraction rate) may be more useful than differential single-Doppler velocities in the characteristic velocity couplet for detecting and measuring the strength of convergent tornadic mesocyclones at low altitudes, because circulation and areal expansion rate are (a) less scale dependent, (b) more tolerant of noisy Doppler velocity data, and (c) relatively insensitive to range and azimuth, beamwidth and location of a tornado within a sampling volume.

At the conference, we will assess estimates of simulated low-level supercell circulation and areal contraction rate diagnosed from virtual Doppler radar data. The supercell simulation was generated using the Advanced Research Weather Research and Forecasting (WRF-ARW) model with 111-m horizontal grid spacing and typical cloud model settings. The 111-m grid was run concurrently within a 333-m simulation in a one-way nested configuration. A tornado develops ~110 min into the simulation and becomes very intense with surface winds briefly exceeding 110 m s^-1 (EF5).

Reference:

Wood, V. T., and R. P. Davies-Jones, 2015: Circulation and areal contraction rate as detected and measured by Doppler radar. Extended abstracts, 37^th Conference on Radar Meteorology, Norman, OK, Amer. Meteor. Soc., P103.